Mechanistic understanding of [Rh(NHC)]-catalyzed intramolecular [5 + 2] cycloadditions of vinyloxiranes and vinylcyclopropanes with alkynes.

Abstract

Zhang and co-workers have recently advanced the carbon [5 + 2] cycloaddition of vinylcyclopropane and alkyne (VCP-alkyne) to the hetero-[5 + 2] cycloaddition of vinyloxirane with alkyne (VOR-alkyne). Herein, we present a systematic computational study to gain insights into the detailed reaction mechanisms and origins of mechanistic differences of the two types of cycloadditions (all-carbon [5 + 2] cycloaddition vs. hetero-[5 + 2] cycloaddition). Instead of the general mechanism of rhodium-catalyzed VCP-alkyne cycloaddition that involves cyclopropane cleavage, alkyne insertion and reductive elimination, the rhodium-catalyzed VOR-alkyne cycloaddition occurs via oxidative alkyne-alkene cyclization, oxirane cleavage and reductive elimination. The cycloaddition of VOR-alkyne represents the first example of preferring the oxidative alkyne-alkene cyclization mechanism within rhodium-catalyzed [5 + 2] cycloadditions. The origins of the mechanistic difference are derived from the stabilizing effects due to the favorable ligand-substrate C-H/π dispersion interaction and the substrate-substrate C-HO hydrogen-bond interaction in the oxidative alkyne-alkene cyclization step of the hetero-[5 + 2] cycloaddition. The VOR-alkyne cycloaddition gives a bicyclo[5.3.0] product featuring a 2,5-dihydrooxepin moiety, which can further undergo a [3,3]-sigmatropic rearrangement giving the final bicyclo[3.1.0] product, because a carbonyl structure is more favorable than an enolate structure.